1. Field of the Invention
The present invention relates to masks which are used to manufacture semiconductor integrated circuits. More particularly, the present invention relates to a half-tone phase shift mask (HT PSM).
2. Description of the Related Art
Semiconductor integrated circuits are made up of predetermined patterns (e.g., a conductive layer pattern, a contact layer pattern, etc.) disposed on a semiconductor substrate. These predetermined patterns are formed from a photosensitive film that is first provided on the surface of the semiconductor substrate. The film is then exposed to light through a mask or a reticle (hereinafter, referred to as a mask) bearing a pattern to be transferred to the photosensitive film. Recently, the design rule of the layers of the integrated circuit has decreased to meet the demand for more highly integrated semiconductor devices. Accordingly, the mask patterns, which are transferred to the photosensitive film during an exposure process, have become hyperfine. However, this trend toward hyperfine mask patterns has given rise to a problem associated with the characteristic of the diffraction of light during the exposure process.
Hence, half-tone phase shift masks (HT PSMs) have received much attention as of late as a means for solving the problems caused by the diffraction of light through a mask bearing a hyperfine pattern. However, the use of an HT PSM in the exposure process cannot completely solve the problems caused by the diffraction of light.
Such problems generated when a photosensitive film pattern is formed using a conventional HT PSM will now be described in detail with reference to FIGS. 1A and 1B. The graph of FIG. 1B is a representation of the diffraction pattern created by a conventional HT PSM, wherein the distance from a reference point on a flat wafer surface is plotted along the horizontal (X) axis, and the intensity of normalized light is plotted along the vertical axis. Furthermore, although the problem created by the use of the conventional HT PSM will be described with respect to the forming of a pattern on a positive photosensitive film, such a problem also applies to forming patterns on negative photosensitive films.
In the conventional HT PSM, as shown in FIG. 1A, a single phase shift pattern 12 is formed on a transparent glass substrate 10, and an opaque pattern 13 is formed on the phase shift pattern 12. The phase shift pattern 12 defines an aperture 14 that exposes a predetermined area of the transparent glass substrate 10. When light is transmitted through the region of the mask delimited by the opaque pattern 13, a phase difference of 180 degrees occurs between light which passed through the aperture 14, and light which passed through the phase shift pattern 12. The width of the aperture 14 defines the width of the photosensitive film pattern.
An image of the pattern of light, namely the diffraction pattern, formed by the HT PSM shown in FIG. 1A is represented by the graph of FIG. 1B. This graph shows a main intensity curve 16 at the center, and a side lobe 18 formed at each side of the main intensity curve 16. At the boundary between the main intensity curve 16 and each side lobe 18 there is a point of inflection where the intensity is shown to be 0. These points of inflection represent locations where destructive interference occurs, i.e., where the wave front of light which passed through the aperture 14 intersects with that of light passing through a portion of the phase shift pattern 12 and out of phase by 180xc2x0 with respect to the light that passed through the aperture 14. It is this phenomena of destructive interference that makes it possible to form a fine pattern on the photosensitive film. Moreover, a fine pattern is only observed when the amplitude of the main intensity curve 16 does not dip below a certain value (representing the point where insufficient exposure of the corresponding photosensitive film portion would occur), and the amplitude of the side lobe 18 does not rise above a certain value (representing the point where unwanted exposure of the corresponding photosensitive film portion would occur).
However, the smaller the width of the aperture 14 becomes in the attempt to increase the integration density of the semiconductor integrated circuit, the more the phase shift pattern 12 provides a negative phase shift effect. That is, the degree to which light is diffracted by the aperture 14 increases as the width of the aperture 14 decreases. The greater the diffraction, the greater the amplitude of the side lobe 18 becomes. Also, the negative phase shift effect extends to the main intensity curve 16, as a decrease in its amplitude.
Accordingly, when using an HT PSM, a great dose of light must be supplied to the photosensitive film to form a satisfactory pattern thereon. Thus, using a conventional HT PSM requires a long exposure time. Also, a great dose of light spoils the profile of the photosensitive film pattern. This is a serious problem for highly-transmissive HT PSMs in which the transmissivity of the phase shift layers are enhanced.
An object of the present invention is to provide a half-tone phase shift mask (HT PSM) that can produce a pattern which, when imaged, is characterized by a main intensity curve having a large amplitude, and yet produces a maximal phase shift effect.
To achieve the above object, the half-tone phase shift mask of the present invention includes a transparent substrate, a phase shift pattern having a stepped aperture that exposes a predetermined portion of the transparent substrate having a width corresponding to the width of a pattern to be formed on photosensitive film disposed on a semiconductor substrate, and an opaque film pattern formed on the upper surface of the phase shift pattern. The stepped aperture is defined by an interior side wall of the phase shift pattern. This side wall includes a horizontal surface which is parallel to the surface defining the bottom of the aperture. Light transmitted by the mask via the surface defining the bottom of the aperture has a phase difference of 180 degrees with respect to light transmitted by the mask via the horizontal surface, and light transmitted by the mask via the surface defining the bottom of the aperture has a phase difference of more than 180 degrees with respect to light transmitted by the mask via the upper surface of the phase shift pattern.
The bottom of the aperture can extend into the transparent substrate from the upper surface thereof. In this case, the phase shift pattern can consist of a single layer. For instance, the phase shift pattern can be formed of amorphous carbon, MoSiON, SiN or spin on glass (SOG).
Alternatively, the bottom of the aperture can coincide with the upper surface of the transparent substrate. In this case, the phase shift pattern can consist of a single layer or of a first layer and a second discrete layer formed on the first layer.
When the phase shift pattern is formed of first and second layers, the first layer of the phase shift pattern defines the width of the bottom of the aperture. The second layer of the phase shift pattern delimits the horizontal surface of the stepped side wall. In one embodiment, the first layer can cause a phase variation of 180 degrees, and the second layer can cause a phase variation of more than 0 degrees in light transmitted therethrough during the exposure process. In this case, the second layer of the phase shift pattern delimits the horizontal surface of the stepped side wall at the top surface of the first layer to form the step in the aperture. In another embodiment, the step can be formed in the second layer itself. In this case, the first layer causes a phase variation of less than 180 degrees.
Still further, the first layer can be of a material having substantially the same refractive index as the transparent substrate.